At a Glance

Why Get Tested?

To help evaluate thyroid gland function; to diagnose thyroid disease, including hyperthyroidism, and determine the cause; to monitor effectiveness of treatment of a thyroid disorder

When to Get Tested?

When you have an abnormal TSH, particularly with a normal free T4 result, and/or signs and symptoms of hyperthyroidism

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

None needed; however, certain medications can interfere with the free T3 and total T3 tests, so tell your health practitioner about any drugs that you are taking.

The Test Sample

What is being tested?

Triiodothyronine (T3) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called thyroxine (T4) and together they help control the rate at which the body uses energy. Almost all of the T3 (and T4) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. Tests can measure the amount of free T3 or the total T3 (bound plus unbound) in the blood.

T3 and T4 production is regulated by a feedback system. When blood levels of thyroid hormones decline, the hypothalamus releases thyrotropin releasing hormone, which stimulates the pituitary gland to produce and release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce and/or release more thyroid hormones. Most of the thyroid hormone produced is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.

If the thyroid gland produces excessive amounts of T4 and T3, then the person affected may have symptoms associated with hyperthyroidism, such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes. In some cases, the person's eyes cannot move normally and they may appear to be staring. In other cases, the eyes may appear to bulge.

If the thyroid gland produces insufficient amounts of thyroid hormones, then the person may have signs and symptoms associated with hypothyroidism and a slowed metabolism, such as weight gain, dry skin, fatigue, and constipation. The blood levels of thyroid hormones may be low or high due to thyroid dysfunction or rarely due to insufficient or excessive TSH production related to a pituitary disorder.

The most common causes of thyroid dysfunction are related to autoimmune disorders. Graves disease causes hyperthyroidism, but it can also be caused by thyroiditis, thyroid cancer, and excessive production of TSH. The effect of these conditions on thyroid hormone production can be detected and monitored by measuring the free T3 or sometimes total T3.

The Test

How is it used?

A free or total triiodothyronine (free T3 or total T3) test is used to assess thyroid function. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor treatment of a person with a known thyroid disorder.

Most of the T4 and T3 circulates in the blood bound to protein, while a small percentage is free (not bound). Blood tests can measure total T4 (unbound plus bound), free T4, total T3 (bound plus unbound), or free T3.

Since most T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not. However, some professional guidelines recommend the total T3, so either test may be used to assess thyroid function. For example, free T3 or sometimes total T3 may be ordered along with thyroid antibodies to help diagnose Graves disease, an autoimmune disorder that is the most common cause of hyperthyroidism.

The free or total T3 test is usually ordered following an abnormal TSH, particularly if the free T4 test is not elevated.

When is it ordered?

A free T3 or total T3 test may be ordered when someone has an abnormal TSH test result. It may be ordered as part of the investigative workup when a person has symptoms suggesting hyperthyroidism, especially if the free T4 level is not elevated.

What does the test result mean?

Increased or decreased thyroid hormone results indicate that there is an imbalance between the body's requirements and supply, but they do not tell the health practitioner specifically what is causing the excess or deficiency.

The following table summarizes some examples of typical test results and their potential meaning.

If someone is being treated with anti-thyroid medication for hyperthyroidism and the free or total T3 (or more frequently, the free T4 or TSH) is normal, then it is likely that the medication is effective in treating the condition. If the free or total T3 or free T4 is elevated, then the medication is not effective in treating the condition and the person may be experiencing symptoms associated with hyperthyroidism.

Is there anything else I should know?

It is generally recommended that thyroid testing be avoided in hospitalized patients or deferred until after a person has recovered from an acute illness since thyroid hormone levels may be affected the stress of an illness. When someone is sick, the body decreases production of T3 from T4. Most people who are sick enough to be in the hospital will have a low T3 or free T3 level. For this reason, health practitioners usually only order T3 tests in outpatient settings.

It is important to note that thyroid tests are a "snapshot" of what is occurring within a dynamic system. An individual person's total T3, free T3, total T4, free T4, and/or TSH results may vary and may be affected by:

Increases, decreases, and changes (inherited or acquired) in the proteins that bind T4 and T3

Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T3 test results and their use should be discussed with a health practitioner prior to testing. In general, free T3 levels are not affected by these medications.

2. What is the T3 uptake test?

This test was once used to help calculate the Free Thyroxine Index (FTI), an estimation of the free T4 concentration. It is determined from the total T4 test and some estimation of the level of thyroid hormone binding proteins. The T3 uptake test was the original test for estimating the level of binding proteins, and later versions were called T-uptake methods. These are rarely used now that there are methods available to measure free T4 and free T3 directly.

3. What is reverse T3?

Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when the liver converts T4 to T3, it also produces a certain percentage of RT3. When the body is under stress, such as during a serious illness, it tries to prevent many tissues that depend on T3 from being metabolically active by producing more RT3 than T3. This is believed to be a way of conserving energy until the stress is relieved and it causes a syndrome called non-thyroidal illness (NTI). RT3 may also be elevated in hyperthyroidism. Use of the RT3 test remains controversial and it is not widely requested.

Article Sources

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

(Updated 2014 May 14). Thyroid Tests. National Endocrine and Metabolic Diseases Information Service [On-line information]. Available online at http://endocrine.niddk.nih.gov/pubs/thyroidtests/index.aspx through http://endocrine.niddk.nih.gov. Accessed June 2014.

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This article was last reviewed on November 3, 2014. | This article was last modified on March 11, 2015.

The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.

The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.